1. Field of the Invention
The present invention relates to a yoke for a universal joint and a production process for the same.
According to the present invention a yoke for a universal joint and a a yoke for a universal joint which is, for example, to be incorporated into a joint portion between a pair of steering shafts for forming a steering apparatus, can be attained by simple press working at low cost.
2. Related Background Art
In a joint portion between the steering shafts of a steering apparatus, there is provided a universal joint 1 as roughly shown in FIG. 7 so that a pair of steering shafts 2, 2 not existing on one straight line are connected with each other to enable transmission of rotational force between these two steering shafts.
Such universal joint 1 is formed by connecting a pair of yokes 3, 3 to each other by one cross shaft 4 so as to allow rockable displacement thereof. Each of these yokes 3, 3 has a pair of arms 5, 5 formed in a U shape and opposite to each other. Mutually concentric circular holes 6, 6 are formed at the tip ends of these arms 5, 5. Inside each of these circular holes 6, 6, the end portion of said cross shaft 4 is supported by a radial needle bearing to allow rockable displacement.
In order to reduce the cost of the universal joint 1 which is described above, each of the yokes 3, 3 is produced by press working of a steel sheet. When the yoke 3 is to be produced by press working, a steel sheet which has been punched into a predetermined shape is bent to a U shape to form the pair of arms 5, 5, and then the above-mentioned circular holes 6, 6 are formed at the tip ends of each of these arms 5, 5. In order to sufficiently reduce the cost of the yokes 3, 3, it is preferable to carry out the work for forming the circular holes 6, 6 at the tip ends of each arm 5, 5 by press punching, and not by, for example, cutting work using a drilling process.
On the other hand, in order to maintain the performance of the universal joint 1, it is necessary to sufficiently increase the coaxiality, the circularity, the inclination, the size accuracy including that of the inner diameter, and the configuration accuracy of the circular holes 6, 6. Because a bearing cup for forming the radial needle bearing is fitted in and fixed to each of these circular holes 6, 6, if the accuracy of any of the above-mentioned items of the circular holes 6, 6 is not sufficiently high, the support portion of the cross shaft 4 may rattle, or this support portion 4 may not rock smoothly, or, a sealing performance of the radial needle bearing portion is deteriorated, which is not desirable since the operability or durability of the steering apparatus incorporating the universal joint 1 is worsened.
FIG. 8 shows a state in which the circular holes 6, 6 are formed at the tip ends (the upper end portions of FIG. 8) of the arms 5, 5 of the yoke 3 by normal press working. The formation of these circular holes 6, 6 by press is conducted by piercing a pair of punches (punchers) into the tip ends of these arms in the state that a die (counter punch) is kept inserted between the paired arms 5, 5. Of the circular holes 6, 6 formed by such normal press working, cylindrical surfaces 8, 8 having comparatively high accuracy are formed on the outer half surfaces into which the punching punches are to be inserted. On the other hand, on the inner half surfaces thereof which are the sides opposite to the die, there are provided conic concave break planes 9, 9 which are unstable in configuration, dimensions, and roughness. The performance of the universal joint having by the yoke 3 in which the circular holes 6, 6 including such break planes 9, 9 (with low accuracy) are formed is poor, as described above.
On the other hand, conventionally, in order to form accurate holes by press working, the punching punch is pressed against the die while environs of the circular holes to be formed are restrained by a part of the steel sheet to be processed in the direction of the thickness. Further, it is known that the accuracy of the circular holes to be formed by press working can be securely obtained by adjusting a clearance between the punching punch and the die, or contriving the configurations of the edge portions of the punching punch and the die.
However, in the case of the universal joint 1 to be incorporated in a steering apparatus, the width W7 of the tip end partition walls 7, 7 provided between the tip end outer periphery of the arm 5, 5 and the peripheral portion of the circular hole 6, 6 is made to be small in order to reduce the size and the weight of the joint. For this reason, it is difficult to securely obtain a sufficient force for restraining the peripheral portions of these circular holes 6, 6 when the circular holes 6, 6 are punched. That is, if this force is increased, a great interfacial pressure acts in the direction of the thickness of each of the tip end partition walls 7, 7 having the small width W7, so that the thickness T7 of each of the tip end partition walls 7, 7 deviates from the regulation value.
Since the above-mentioned restraining force can not be securely obtained for such reason the coaxiality, the circularity, the inclination, and the size accuracy including that of the inner diameter of the circular holes 6, 6, and thus the performance of the universal joint 1 formed by the yokes 3, 3 with the circular holes 6, 6 provided at the tip ends thereof are deteriorated. Accordingly, it is difficult to actually adopt such universal joint in a steering apparatus of a motor vehicle.
A yoke for a universal joint and a production process of such yoke according to the present invention were contrived to utilize punching work by press working at low cost and to prevent the deterioration of accuracy of the circular holes which is associated with the deterioration of performance mentioned above, thereby being broadly suited to the production of a steering apparatus of a motor vehicle.
A and after for a yoke a for universal joint according the present invention is produced by press forming of a steel sheet, in the same manner of a conventionally-known yoke for universal joint, and mutually concentric circular holes are formed by press punching at the tip ends of a pair of arms opposite to each other.
In the yoke for a universal joint of the present invention, after said circular holes are provisionally by inserting a punch through the tip ends of said arms in the state that the backs of said arms are supported by a die relative to the punching direction and the outer peripheries of these arms are restrained by a restraining die in the vicinity of a portion in which said circular holes are to be formed, a finishing punch is inserted from the direction opposite to the first memory punch into the provisionally punched circular holes, thereby improving the accuracy of said circular holes.
Also, a production process of a yoke for universal joint according the present invention includes a question to produce, after forming a pair of arms opposite to each other by bending a steel sheet by press forming, mutually concentric circular holes by press punching at the tip ends of these arms, also in the same manner as a conventionally-known production process of a yoke for universal joint.
In the production process of the present invention after provisionally punching said circular holes by inserting a punching punch through the tip ends of said arms in the state that the backs of said arms are supported by a die relative to the punching direction and the outer peripheries of these arms are restrained by a restraining member in the vicinity of a portion for forming said circular holes, a finishing is inserted from the direction opposite to the first mentioned punch into the provisionally punched circular holes, thereby improving the accuracy of said circular
According to the yoke for universal joint and the process of producing such yoke of the present invention arranged as described above, the punching work is conducted by a low-cost press working so as to prevent the deterioration of accuracy of the holes which is associated with deterioration of performance mentioned above. As a result, an inexpensive universal joint with high performance can be obtained.